1. Field of the Invention
The present invention relates to a liquid crystal flat panel color display, and more particularly, to a flat panel color display which includes a light source, a prism, a layer of metal film, a layer of liquid crystal, an array of transparent electrodes, a glass display screen, and a controller for controlling the electrodes to selectively vary the dielectric constant of the metal-liquid crystal interface. The variance of the dielectric constant changes the plasmon resonance condition and controls the color of light directed to the display screen through the selective scattering of incident white light by surface plasmons.
2. Summary of Related Art
In recent years, a considerable amount of research has been conducted in an attempt to develop a low profile, full color electronic display system which does not rely upon conventional cathode ray tube technology. Such "flat panel screens" are used in a variety of applications, including lap top computers and aviation instrument panels. Additional applications are being developed, including computer terminals, automobile instruments, high definition television and medical instruments.
At this time, there are a number of cost, production, and design difficulties which are limiting the use of flat panel screens in the various applications. New flat panel screen technologies are needed to reduce the cost of, improve the performance of, and simplify the manufacturing process for the flat panel screens. Because flat panel screens have space and layout advantages over the present cathode ray tubes, the market for flat panel screens will continue to grow as the improved flat screen technology overcomes the cost and production difficulties. Cathode ray tubes typically have large depth dimensions and consequently occupy a considerable amount of floor or counter space. Because cathode ray tubes require an elongate neck to provide for the acceleration of an electron beam from the electron gun to the face plate of the cathode ray tube, the tubes are quite irregular in shape. Cathode ray tubes are constructed from relatively thick glass and are inordinately heavy, extremely fragile and easily breakable. Cathode ray tubes also require a relatively high voltage power supply to accelerate the electron beam and sustain the displayed image, and such a power supply is not typically available in portable applications.
In a cathode ray tube, an electron beam is deflected through a prescribed angle as it leaves the gun. The electron beam strikes the inside of the glass screen, which is coated with a phosphorescent dot material that emits light when struck by the electron beam. The electron beam is swept rapidly and repeatedly across the cathode ray tube screen to produce a screen of varying intensity rather than a sequence of discrete dots. In a black and white tube, white dots are used and the luminance is varied to achieve shades of gray. In a color tube, three different color phosphorescent dots (red, blue, and green) are used to achieve the various colors, including white.
The color picture tube contains three separate electron guns that produce three different scanning beams simultaneously. The beam from the red gun strikes a red phosphor dot on the tube to cause the dot to glow red. Blue and green dots are struck by their respective electron beams. The viewer, at a distance from the display screen, sees a single color that is a combination of red, blue, and green. The dots are arranged in a two-dimensional matrix field of the display screen.
The term "pixels" is used herein as a generic term for picture elements or picture dots in a display screen, such as the phosphorescent dots in the cathode ray tube, which are arranged in the rows and columns of the matrix. A high definition color screen may have over one million pixels (a matrix of 1024 pixels by 1024 pixels).
A number of different technologies are being developed to overcome the space problems of the cathode ray tube and to facilitate the continued development of the flat panel displays. Liquid crystal, gas plasma, laser/LED, and field-emitters are presently considered the most promising.
In the gas plasma devices, an electric current causes a miniature tube of gas at each pixel to glow. The laser/LED devices utilize miniature laser beams or light emitting diodes to directly illuminate the individual pixels. In a field-emitter display, arrays of microscopic cathodes are aimed at a phosphor dots on the screen to cause the dots to glow.
The primary form of flat panel screen in use at this time is the active matrix liquid crystal display. Liquid crystal displays are typically either reflective or transmissive. A reflective liquid crystal display depends upon ambient light conditions in order to be viewed. Light from the surrounding environment incident upon the side of the display facing the viewer is reflected back to the viewer. Differences in the orientation of the liquid crystal material within each liquid crystal pixel cause those pixels to appear either darkened or transparent. Reflective liquid crystal applications are severely limited because they cannot be used in a dark or low light environment.
Transmissive liquid crystal displays require the use of one or more back lights. Transmissive liquid crystal displays disclosed in the prior art still utilize the three color system of phosphorescent dots. A liquid crystal surface acts as a shutter to block light or allow light to pass through the layer of liquid crystal to lighten each pixel (phosphorescent dot) on the screen. A transistor-electrode at each pixel activates the liquid crystal shutter. The present invention is directed to the field of backlit liquid crystal displays in which the phosphorescent dots are not utilized and color is provided by the scattering of color light in the surface plasmon mode.
The liquid crystal, gas plasma, and field-emitters technologies are similar in that they all rely on a matrix of electrodes. Electrical signals applied to the matrix of electrodes control a working medium. Commonly used working mediums include liquid crystals, neon and phosphors. The working medium is typically sandwiched between the matrix of electrodes. The light emitted from a screen pixel is regulated by energizing its associated electrodes. If the proper electrical signals are rapidly applied to the electrodes, still and moving images can be formed.
To achieve a colored screen in a cathode ray tube display, three separate electron guns are used to energize phosphorescent dots on the inside of the display screen. The dots are arranged in an alternating color pattern in order to facilitate the combination of the blue, red, and green dots to form the various colors on the screen.
In flat panel displays, the color systems have not reached the level of color performance of cathode ray tube displays for color television. Color filters and color screens have been used to provide the combination of blue, red, and green dots needed for color displays. Wave guides are often used to collimate light and to transmit light to the desired locations on a display screen. Color filters may be used to limit the color of light being transmitted in a wave guide. The "filtering" of light into one of the three primary colors prior to entry into the wave guide permits a display screen to utilize a plurality of wave guides with filtered light to display color images. The control and operation of the color displays is very complex for flat panel applications, and researchers have been working to develop an improved color display system for flat panel applications.
In the design of flat panel displays having a back light system, there are a number of design features and specifications which must be addressed to provide a viable flat panel display. The lighting intensity should be uniform across the large surface areas illuminated by the back light. The intensity of the light must be substantially the same at each pixel of the liquid crystal display. The back light must also provide high brightness illumination to yield a sharp image to a remotely positioned viewing audience. The back light system must maintain a low profile, which requires the liquid crystal display to be substantially flat.
The design of the back light system should take into consideration the number and configuration of the back lights. Heat is often a problem in flat panel displays. Power consumption is also a concern, especially in portable systems which are battery operated. Improving the efficiency of the back light system for a liquid crystal display helps to reduce heat generation and power consumption.
U.S. Pat. No. 4,992,916 to Henkes discloses an illuminator for a flat panel display which includes at least one light source and at least one prism having total internal reflection at all but its front surface. The light source is located within a prism to provide a thin and efficient optical cavity. The total internal reflection provides multiple images of the associated light source at the front surface.
U.S. Pat. No. 5,083,120 to Nelson shows a flat panel display utilizing an array of sequentially illuminated leaky guide lights as the row-back light component of the display. Light is directed into the guide and is designed to leak light in a uniform manner along the longitudinally extending edge of the guide.
Wave guides for flat panel screen applications are disclosed in U.S. Pat. No. 5,106,181 to Rockwell. Fiber optic wave guides are placed in parallel across the substrate to determine either the row or column resolution of the display. The wave guide is tapped to cause the light to exit the wave guide and illuminate the screen. Filters are used to convert white light into color light to make a full color display.
U.S. Pat. No. 5,161,041 to Abileah et al. teaches a lighting assembly for a backlit flat panel display. The lighting assembly includes an integrally formed image splitting and collimating lens for enlarging the area illuminated by the lamps.